Dewatering is a process in which liquid is separated from solid with no accompanying phase change of the liquid. Commonly, mechanical dewatering apparatus is used for such purposes, examples including belt press filters, filter presses, roller presses, vacuum filters or centrifuges. Electrical dewatering techniques are also available, such as electro-osmosis, or the use of magnetic fields. Electro-osmosis is the phenomenon of liquid moving through a porous medium under the application of a direct current electric field. Under these circumstances the positive metal ions can migrate to the negatively charged cathode, transporting water with them probably via viscous interactions, molecular collisions and/or as a hydration sheath. This technique is particularly effective for dewatering materials which are difficult to dewater using mechanical pressure alone such as fine or colloidal particle suspensions, sewage sludge, silt sludge or gelatinous materials. However, these solid-liquid mixtures, filter cake permeability is often very poor. Negatively charged particles will be repelled by the cathode, thereby reducing fine particle clogging of any associated filter medium and allowing better drainage of the interstitial fluid past that cathode.
By combining the driving forces for dewatering from both mechanical and electrical sources a cumulative effect can be achieved which is important in the search for a higher efficiency solid/liquid separation process. Parameters such as filtration rate, final cake moisture content and power consumption are taken into consideration.
U.S. Pat. No. 5,401,375 (Yamaguchi et al) describes a revolving anode and a moving filter belt positioned over a taut metallic cathode, the system being designed to apply electro-osmosis and mechanical compression simultaneously. It is important with such equipment that the maximum pressure and maximum electric field occurs at the same locations with only a very small distance between anode and cathode to ensure a low voltage drop and high electrical efficiency. In U.S. Pat. No. 5,401,375 various embodiments are described wherein a thin sludge-receiving space is provided between a rotary drum anode and a water transmissive press belt which functions as a cathode. Electrode elements are provided for direct contact with the sludge; these elements are in the form of an anode wire or plate depending on the embodiment. An electric field applied over an extensive area is thus provided and Yamaguchi et al describes methods for dealing with water and gas at the anode which is said to occur due to electrolysis. Water in the sludge is, however, discharged towards the cathode through a filter cloth belt which supports the sludge on top of a press belt; the belt is connected as a cathode and has drain holes. Yamaguchi et al suggests that this arrangement is preferable to earlier prior art arrangements in which insulating belts cover both anode and electrode surfaces and significantly reduce conductivity between the sludge and the electrodes.
In a similar application, ZA910538 describes apparatus which can apply electro-osmosis and mechanical compression by means of two endless belt electrodes, the anode belt being constructed from carbon fibres or electrically conductive synthetic materials, and the cathode belt consisting of a belt made of metallic mesh, with a second belt of filter cloth material to support the resultant filter cake located between the feed slurry and the cathode belt. In use the sludge is squeezed between the two belts. Once again, no details are provided of the particular construction of the water transmissive belt cathode. Further, the layer of filter cloth acts as a significant insulating barrier, reducing the conductivity able to be maintained between the sludge and electrode belts.
Similarly, U.S. Pat. No. 5,891,342 describes dewatering in a flocculated sludge using a compressive belt filter containing electrically conductive material, where the belt is comprised across its width of a plurality of connected spiral yarns, the yarns being made of polyester or polyamide helical coils and the electrically conductive material is merely woven or inserted into these spiral yarns. U.S. Pat. No. 5,891,342 describes how the belt may wholly comprise electrically conductive material inserted into the base material of the belt, extending right through the belt as well as being located in each coil. The electrically conductive material comprises wires or strips needled to one or both sides of the belt, so that the belt is fully water transmissive across its width. However, a tendency in this sort of apparatus is for the electrical current to short-circuit via the edge of the belt to other parts of the apparatus, and bypass the wet filter cake.
In further examples available in the art, industrial belt filters have been adapted for electro-osmosis by placing a perforated cathode below the filter belt, and making use of a stainless steel pressure plate at the topside of the sludge as an anode. U.S. Pat. No. 4,861,496 describes an anode in such equipment which features metal bristles or wires which protrude into the sludge mass, with a cathode below the filter belt but the form of the arrangement is not described. In such a case the anode design is of principal interest.
Other devices exist where a revolving anode has a moving filter belt pass between it and a revolving cathode, the two electrode drums being used to apply electro-osmosis and mechanical compression simultaneously, but where the filter belt is manufactured of a non-conductive fabric.